This invention relates to riser systems, and relates more particularly but not exclusively to riser systems for use in maritime installations wherein a flexible conduit extends between a seabed location and a floating vessel.
It is common practice for submarine wells producing hydrocarbons to have a wellhead mounted on the seabed, with the hydrocarbons recovered from the well being fed to a surface-floating vessel by way of a flexible hose. Such vessels are commonly anchored to the seabed to retain them in suitable proximity to the respective wellheads without the continuous expenditure of energy necessary for dynamic positioning. Nevertheless, moored vessels are not totally static, since they are subject to winds, waves, currents, and tides, with consequent changes in alignment, heading, depth, and position. Thus the point at or near the upper end of the hose where the hose is directly or indirectly tethered to the surface-floating vessel is subjected to considerable tension, and must also accommodate changes in the direction of tension.
It is an object of the invention to provide a riser system in which changes in the direction of tension at or near the upper end of the hose or other form of riser are at least partially accommodated by means other than inherent flexibility of the hose or other form of riser.
As used in the specification and in the appended claims, the term xe2x80x9csurface-floating vesselxe2x80x9d encompasses semi-submersible floating vessels, and extends to static structures (including non-floating vessels and structures) coupled to one or more risers subjected to directionally variable tensions, e.g. by reason of variable currents or tidal flows, since the invention is applicable in such circumstances.
As used in this specification and in the appended claims, the term xe2x80x9criserxe2x80x9d encompasses flexible hoses, flexible conduits, flexible umbilicals, flexible tethers, flexible cables, flexible mooring elements, and functional equivalents thereof in the form of a plurality of relatively short lengths of relatively rigid materials or members mutually linked by articulating joints; i.e. the xe2x80x9criserxe2x80x9d can have any suitable physical form, and may be intended to carry fluids, or the riser may simply be a tension-transmitting member not intended to transport fluids. Equally, the riser may be a steel pipe.
According to the present invention there is provided a riser system comprising a riser which extends from a lower end location to an upper end location whereat the riser is tethered by .a tether means comprised in the riser system, the tether means comprising a riser clamp means and a pivot means coupling the riser clamp means to an anchorage, the pivot means having at least one degree of rotational freedom.
Said at least one degree of rotation freedom preferably comprises freedom to rotate about a substantially horizontal axis, the substantially horizontal axis preferably being substantially orthogonal to a substantially vertical plane including the catenary or other shape formed by the riser under the composite influences of gravity and flotation.
The pivot means may have a second degree of rotational freedom, preferably comprising freedom to rotate about a substantially vertical axis.
The riser clamp means may comprise an elbow means to which the riser is clamped against longitudinal movement while maintaining through passage of fluid where the riser is a hose or other fluid conduit, or the riser clamp means may comprise a termination for the riser, the termination being constructed or adapted to maintain through passage of fluid from the riser where the riser is a hose or other fluid conduit. In either case, where the riser is a hose or other fluid conduit, the riser clamp means is preferably coupled by a further flexible or articulated hose or other fluid conduit to static pipework downstream of the riser system.